Since particulate matters (PMs) which are made of carbon fine particles, SOF (Soluble organic Fraction), sulfate, and the like are contained in exhaust gas exhausted from diesel engines, these PMs must be removed from the exhaust gas in order to exhaust purified exhaust gas. Since the PMs can be hardly removed by the normal oxidizing catalyst, 3-way catalyst, and the like, generally speaking, after the PMs are collected by filters (DPFs) and oxidized to be removed.
As such a DPF, wall flow type DPFs have been widely utilized. In one wall flow type DPF, an inlet-side cell clogged at a downstream-side edge portion, and also an outlet-side cell clogged at an upstream-side edge portion and located adjacent to the inlet-side cell have been formed in a honeycomb body. This honeycomb body owns a large number of cells, and is made of thermal resistant ceramics such as cordierite. In this DPF, exhaust gas which has entered to the inlet-side cell passes through a cell wall and then is exhausted from the outlet-side cell. While the exhaust gas passes through the cell wall, the PMs are collected in pores of the cell wall. Thereafter, when certain amounts of the PMs are collected, the collected PMs are burned by heating these PMs by a heater, so that the DPF function may be reproduced.
However, in such DPFs, heat generation amounts are increased by the combustion when the DPF functions are reproduced in such a case that PM collecting amounts are large. Thus, there are some possibilities that these DPFs are damaged by heat shock. Also, manufacturing cost as to such DPFs is high. As a consequence, several ideas as to DPFs made of metals have been recently proposed.
For instance, JP-A-9-262414 describes such a DPF in which while a corrugate plate made of a metal thin plate and a plain plate made of metal non-woven cloth are alternately stacked with each other, both an inlet-side cell clogged at a downstream-side edge portion and an outlet-side cell clogged at an upstream-side edge portion and located adjacent to the inlet-side cell are formed. Also, JP-A-2002-113798 describes such a DPF in which while a corrugate plate and a plain plate made of metal non-woven cloth are alternately stacked with each other, both an inlet-side cell clogged at a downstream-side edge portion and an outlet-side cell clogged at an upstream-side edge portion and located adjacent to the inlet-side cell are formed.
In accordance with these DPFs, PMs contained in exhaust gas are collected into the metal non-woven cloth. Then, even when the reproducing process operations for burning the PMs by heating manners are carried out, since the DPFs is made of metal, heat shocks are low and damages can be suppressed. However, since any of these DPFs correspond to the wall flow type DPFs, exhaust gas pressure losses are increased in connection with collecting of the PMs. Moreover, since the PMs are concentrated to be deposited in the vicinity of the clogging portions of the inlet-side cells, there is such a problem that the exhaust gas pressure losses are increased rapidly. In the case that engine efficiencies, fuel consumption, and the like constitute major factors, the reproducing process operations must be frequently carried out.
On the other hand, German Utility Model NO. 20,117,873 U1 discloses a filter in which a corrugate plate made of a metal foil and a filter layer are alternately stacked with each other; a plurality of nail-shaped holes having nail-shaped heights are formed in the corrugate plate; the plural nail-shaped holes form a passage which has an inner-directed nail-shaped hole and an outer-directed nail-shaped hole; the inner-directed nail-shaped hole and the outer-directed nail-shaped hole are arranged in such a manner that these nail-shaped holes define an angle; heights of the nail-shaped holes are equal to 100% to 60% of a structural height; and at least 20% of a flow free degree is guaranteed.
In accordance with this filter, exhaust gas derived from the nail-shaped holes pass through the filter layer, so that PMs are collected by the filter layer. Also, since this filter is mainly formed by way of the straight flow structure, increasing of the exhaust gas pressure loss may also be suppressed. Then, since this filter is made of the metal, there is no damage due to thermal stress. However, in this filter, when the depositions of PMs on both the filter layer and the portions of the nail-shaped holes are progressed, the passage of the exhaust gas is blocked, so that the exhaust gas pressure loss is rapidly increased. As a result, since the amount of the collectable PMs cannot be increased, there is a drawback that the PM collecting efficiency must be lowered.
Also, filter catalyst to which catalyst functions have been applied has been developed. For example, JP-A-9-262415 describes the following filter element. That is, a plain plate filter and a corrugate plate are alternately overlapped with each other, and then, the overlapped filter/corrugate plate is wound so as to construct a pillar-shaped body. Then, both edges of this pillar-shaped body are alternately filled to constitute the filter element. The plain plate filter is manufactured by that either ceramics or a metal is filled into either a 3-dimensional mesh-shaped structural porous body or holes of this porous body so as to essentially reduce diameters of these holes. The 3-dimensional mesh-shaped structural porous body is made of a thermal resistant metal and owns a continuously evacuated hole. In this filter element, the catalyst metal is carried on either the corrugate plate or the plain plate element.
In accordance with this filter element, the PM trap and the catalyst converter can be formed in an integral body, resulting in a spatial saving merit. Also, since the carrier member is made of the metal, the heat capacity is small, the temperature increasing speed of the catalyst metal is increased, and such a temperature required for effectively actuating the catalyst metal can be easily obtained. Furthermore, in such a case that such a NOx sorbing material as an alkali metal is carried as the catalyst metal, the below-mentioned merit may be obtained. That is, if a base is made of cordierite or the like, then this base may be reacted with the NOx sorbing material. However, if a base is made of a metal, then this metal base may not be reacted with the NOx sorbing material.
However, in accordance with the technical idea disclosed in JP-A-9-262415, since the 3-dimensional mesh-shaped structural porous body is employed which is made of the thermal resistant metal and has the continuously evacuated hole, the diameters of the holes are made random. In order to carry catalyst, it is effective to form such a catalyst layer containing a porous oxide which has carried a catalyst metal. However, it is practically difficult to form a uniform catalyst layer on the 3-dimensional mesh-shaped structural porous body made of the heat resistant metal, the hole diameters of which are made random. There are various problems. That is, the exhaust gas pressure loss is largely increased, activity caused by the catalyst metal cannot be sufficiently obtained, and grains of the catalyst metal are grown due to heat produced when the filter element is used, so that the activity is lowered.
For instance, JP-A-2001-241316 discloses an exhaust gas purifying apparatus in which one pair of DPFs are arranged in a series manner, and such a DPF having a straight passage, both ends of which are not clogged, is arranged on the upstream side. In accordance with this exhaust gas purifying apparatus, since a portion of the exhaust gas is directly entered from the straight passage to the DPF of the downstream side, this DPF of the downstream side is quickly heated, so that the PMs collected by the DPF of the downstream side can be burned. As a consequence, while the collecting efficiency of the PMs is not lowered, the combustion efficiency of the PMs can be improved and the combustion of the PMs can be finished within a short time.
Moreover, in the recent year, as described in, for example, JP-A-9-173866, a continuous reproducing type DPF (filter catalyst) has been developed, in which a coat layer made of alumina is formed on a surface of a cell wall of the DPF, and such a catalyst metal a platinum (Pt) is carried in this coat layer. In accordance with this continuous reproducing type DPF, since the collected PMs are oxidized/burned due to catalyst reactions width the catalyst metal, the PMs are burned at the same time when the PMs are collected, or the PMs are continuously burned while the PMs are collected, so that the DPF can be reproduced. Then, since the catalyst reaction may occur at a relatively low temperature and the PMs can be burned under such a condition that the collecting amount of the PMs is small, there are such merits that the thermal stress applied to the DPF is small and the failure of the DPF can be avoided.
Also, JP-A-9-053442 describes an exhaust gas purifying apparatus in which an oxidizing catalyst having the straight flow structure, the above-described filter catalyst, and NOx sorbing-and-reducing catalyst having the straight flow structure are arranged from an upstream side of exhaust gas in this order. As previously explained, since either oxidizing catalyst or the NOx sorbing-and-reducing catalyst is combined with the filter catalyst, the purifying performance as to the PMs and NOx may be improved. For example, such a difficulty may be suppressed. That is, the PMs which are entered to the filter catalyst are oxidized by the oxidizing catalyst on the upstream side and the entered PMs are brought into very fine particles. In this way, the problem that the particles are deposited on the edge plane of the filter catalyst so that the filter catalyst is blocked, can be avoided. Also, since the temperature of the exhaust gas is increased due to the oxidization reaction by the oxidizing catalyst, or NO2 having high oxidation activity is produced, the oxidation of the PMs is emphasized and the purification performance as to NOx may be improved.
In addition, another exhaust gas purifying apparatus is known in which while urea-NOx-selective-reducing catalyst is employed and urea is added into the exhaust gas, NOx is reduced so as to be purified by either urea or produced ammonia by way of the urea-NOx-selective-reducing catalyst. Since this urea-NOx-selective-reducing catalyst is combined with the filter catalyst, HC, CO, and PMs can be oxidized so as to be purified, and NOx can be reduced so as to be purified.
However, in the technical idea described in JP-A-2001-241316, in such a case that the exhaust gas in the low temperature range is continuously entered, it is not possible to avoid such a fact that the PMs are deposited on the upstream-side edge plane of the filter catalyst. Thus, there are some possibilities that the exhaust gas pressure loss is increased since the edge plane is blocked. Also, in the filter catalyst, the coat amount of the catalyst layer is limited in order to avoid increasing of the exhaust gas pressure loss. Under such a temperature condition that the filter catalyst is exposed to higher temperatures than, or equal to approximately 600° C., there are some possibilities that since the particles of Pt are grown, the purifying performance is lowered.
Also, in the case of diesel vehicles, reducing agent such as light oil is added to exhaust gas in an intermittent manner so as to increase reducing performance with respect to NOx. As a consequence, in the case that the technical idea described in JP-A-9-053442 is applied to this system, since the reducing agent is partially oxidized by the oxidizing catalyst of the upstream side, the gasification of this reducing agent may be progressed, and it may be expected that the purifying performance as to NOx may be improved. However, in order to achieve this effect by the oxidizing catalyst, the length of the oxidizing catalyst must be made long. If this length of the oxidizing catalyst is made long, then there is such a problem that the oxidizing performance as to the PMs is lowered, since the filter catalyst is arranged at such a position located far from the diesel engine on the downstream side.
Then, in the exhaust gas purifying apparatus using the urea-NOx-selective-reducing catalyst, since NOx can be hardly reduced in the upstream portion of the urea-NOx-selective-reducing catalyst, the dimension of this urea-NOx-selective-reducing catalyst must be made bulky. Also, in such a case that the urea-NOx-selective-reducing catalyst is combined with the filter catalyst, since the temperature of the exhaust gas cannot be increased in the urea-NOx-selective-reducing catalyst, there are such problems that this combination owns a demerit as to the temperature increase, and the oxidizing activity as to the PMs is low.